1. Field of the Invention
The invention relates generally to control of communication devices in a wireless communication system, and more particularly, to selecting a desired position fix to determine the geographic location of a device in a communication system.
2. Description of the Related Art
Many different types of wireless systems for communications exist, such as cellular networks for telephone communication, broadband Personal Communication Services (PCS), wireless local-area networks (LANs), satellite communication systems, and two-way radio systems. Typically, these wireless systems include fixed infrastructure, having one or more communication devices, and remote units that may be either fixed or mobile. Examples of mobile remote units, also called mobile stations (MS), include cellular telephones, satellite telephones, wireless communication enabled personal computers, personal digital assistants (PDA), and other wireless devices. Fixed remote units can include, for example, wireless local loop (WLL) stations.
Typically, the infrastructure of a wireless system includes one or more BTS transceiver system (BTS) units, at fixed locations, that communicate with the MSs. The BTSs may also communicate with fixed remote units, each other, and/or an external network.
In many situations it is desirable to determine the location of an MS. For example, many location based services (LBS) communicate particular information to an MS user based on the location of the MS. For instance, an LBS may provide an MS user with driving directions to a destination identified by the user based on the user's location, or a LBS may provide information about a current sales promotion if the user is located in close proximity to a store where the sale is taking place. Many other LBS services exist, including, for example, gaming, security, and fleet management.
Another motivation for determining the location of an MS is to support wireless carriers in complying with a U.S. Government Federal Communications Commission (FCC) regulation that requires carriers in the United States to be able to locate the position of a cellular telephone making an emergency “911” call within 50 meters for 67 percent of such calls and within 150 meters for 95 percent of such calls.
There are a variety of different techniques to determine a geographic position fix of an MS in a wireless communication system. Generally, the different techniques fall into two categories, referred to as terrestrial, or network based, and non-network based, which includes satellite-based data.
Terrestrial or network based techniques, in general, involve information about signals transmitted between an MS and the wireless communication system network, or infrastructure with which the MS operates. An example of such network based fixes includes measuring the time of arrival (TOA) of an MS signal received at two different fixed receivers within the network. The difference in the time of arrival of the same transmitted signal at the respective different receivers defines a hyperbola upon which the MS is located.
Another network based fix involves measuring the time difference of arrival (TDOA) of pilot signals, that are synchronized to each other, and that are transmitted from different transmitters in the network to the MS.
In addition, most cellular system infrastructure antennas are sectorized and use a unique transmission signal for each sector. Identifying the unique transmission signal provides information of the angle of arrival (AOA) of the signal at the MS.
Another network based technique measures the round trip delay (RTD) of a signal transmitted from the network to the MS and back to the network. The RTD of a signal provides information of the range of the MS from the network transmitter/receiver.
Non-network based techniques employ information from sources other than from the wireless network. For example, an MS can gather information from satellites of a satellite positioning system (SPS), such as the US global positioning system (GPS), or from land based transmitters of the long range navigation (LORAN) system, or other navigation systems.
If a non-network technique uses GPS, the MS acquires signals transmitted from at least four GPS satellites to determine a three dimensional position fix, or uses at least three satellites to determine a location if an altitude is assumed. By measuring the phase of the received GPS signals, and demodulating the navigation message on the GPS signal to recover the “ephemeris” data defining the location of the satellites, the location of the MS can be determined.
The network and non-network based techniques can be combined in a hybrid technique to improve the determination of the location of an MS. For example, in rural and suburban regions there may not be multiple network transmitters and receivers within range of the MS, so that a network based technique will not provide a position fix. However, in the same rural and suburban regions, a non-network based technique, such as GPS, will usually be able to acquire signals from four or more satellites and can determine a location.
Conversely, in dense urban regions and inside buildings, GPS receivers may not detect a sufficient number of satellite signals to be able to determine a location. However, in these areas there is usually adequate installed network infrastructure so that the MS is within range of two or more BTSs. In other words, the hybrid technique takes advantage of information that is already available to both the MS and the wireless network, i.e. network based techniques, and combines it with navigation information from outside the network, i.e. non-network based techniques, to improve determining the location of the MS.
Different approaches to obtaining a position fix can be implemented to determine the geographic location of an MS in a communication system. The relative performances of the different approaches, as applied to any particular MS and type of wireless communications system, will depend, at least in part, on characteristics of the geographic region where the MS is located. Therefore, it would be desirable to select a position fix based on the performance of the position fix technique in the environment in which the MS is located.